The present invention relates to a monochromator and a spectrometric method for projecting a measured beam (a beam which is measured) on one and the same diffraction grating a plurality of times.
Conventionally, a spectroscope called xe2x80x9cmonochromator,xe2x80x9d has been used as an instrument to measure wavelength characteristics of a measured beam. Particularly, a double monochromator is widely used to allow keeping a high resolution or a wide dynamic range by incidence of a beam into one or more diffraction gratings a plurality of times.
FIG. 9 shows a configuration of a conventional Littrow monochromator. The conventional monochromator shown in FIG. 9 comprises an incident fiber 100, a parabolic mirror 102, a plane diffraction grating 104, an exit slit 106, photodetector 108, an intermediate slit 110, and two return mirrors 112, 114.
In the monochromator shown in FIG. 9, light emitted from the incident fiber 100 is converted into parallel rays by the parabolic mirror 102, and the parallel rays are diffracted by the plane diffraction grating 104. The diffracted beam are returned to the parabolic mirror 102 again, and then returned by the two return mirrors 112 and 114, which are disposed in the vicinity of the focal point of the parabolic mirror 102. Then, the measured beam travels along the same optical path as that along which it has traveled so far in the reverse direction, and is emitted through the exit slit 106, which is disposed in the vicinity of the incident fiber 100, to reach the photodetector 108. In addition, the intermediate slit 110, which has a slit cut in the same direction as the rulings of the plane diffraction grating 104, is disposed between the above mentioned two return mirrors 112 and 114, so that a dynamic range for a wavelength xcex of the diffracted beam passing through the exit slit 106 can be increased. The arrangement of the two return mirrors 112 and 114 in a wavelength sweep direction when the plane diffraction grating 104 is rotated shown in FIG. 9 is referred to as an additive dispersion arrangement.
If the additive dispersion arrangement is realized using the return mirrors 112, 114 and intermediate slit 110 as in the case of the conventional monochromator described above, there are a normal optical path and a reverse optical path both passing through the intermediate slit 110, and the light having traveled along the reverse optical path is a stray light, which reaches the vicinity of the exit slit 106. Therefore, the light observed by the photodetector 108 includes both of the light returned by traveling along the normal optical path and the light returned by traveling along the reverse optical path, thereby generating spurious to cause a problem that the dynamic range is decreased.
FIG. 10 is a partial configuration diagram showing the intermediate slit 110 and two return mirrors 112 and 114 of the monochromator shown in FIG. 9. As shown in FIG. 10, in addition to a normal optical path A, there is a reverse optical path B which is opposite in direction to the normal optical path A in the vicinity of the intermediate slit 110. The light having traveled along the normal optical path A reaches the exit slit 106, and the light returned by traveling along the reverse optical path B also reaches the vicinity of the exit slit 106. Therefore, a wavelength component of the light having reached the photodetector 108 by traveling along the normal optical path A has a spurious of a wavelength component of the light having reached the photodetector 108 by traveling along the reverse optical path B.
The present invention is devised in view of such a problem, and an object of the present invention is to provide a monochromator and a spectrometric method that ensure a wide dynamic range by eliminating a stray light in a reverse optical path.
The monochromator according to the present invention has a return mechanism for returning measured beam which is diffracted by a plane diffraction grating and collected by a collimator, and the return mechanism has return mirrors disposed side-by-side in a wavelength sweep direction, a displacement member for displacing the measured beam in a direction parallel to rulings of the plane diffraction grating, and a cut-off plate disposed in the vicinity of the return mirrors along the normal optical path.
Furthermore, according to the spectrometric method of the present invention, when the measured beam which is diffracted by the plane diffraction grating is to be returned after being collected by a collimator, the beam is displaced by a displacement member in a direction parallel to rulings of the plane diffraction grating and is passed through a cut-off plate disposed in a position along the normal optical path of the measured beam.
It is provided that the measured beam traveling along the normal optical path passes through near the cut-off plate when it is displaced by the displacement member. However, when the measured beam travels along the reverse optical path, this condition is not satisfied, so that the measured beam is cut of f by the cut-off plate. Thus, the stray light, which occurs when the measured beam travels along the reverse optical path, can be prevented, so that it is possible to suppress the occurrence of the spurious and ensure a wide dynamic range.
Especially, it is desirable that the above-mentioned cut-off plate is a first cut-off slit having a slit of a predetermined width formed in a direction perpendicular to the rulings of the plane diffraction grating. Or, it is desirable that an upper side or a lower side of the above-mentioned cut-off plate is disposed in the vicinity of the normal optical path. It is possible to return only the measured beam along the normal optical path and eliminate the stray light along the reverse optical path accurately by the above-mentioned first cut-off slit or the above-mentioned arrangement of the first cut-off slit.
Moreover, it is required that the above-described displacement member is a plate-like member made of a transparent material and a surface of the plate-like member serving as an incidence plane is required to be inclined with respect to a travelling path of the measured beam. When beam is launched into a surface of the plate-like member, the beam is diffracted two times by the surface and a back surface, so that an exit beam parallel to the incident beam is obtained. Thus, since the displacement member can be constituted by a plate-like member having a simple configuration, it is possible to reduce costs of parts.
In addition, it is required that an amount of displacement by the above-described displacement member is larger than the width of the slit formed in the first cut-off slit. Because of this, it is possible to eliminate the measured beam along the reverse optical path accurately by the first cut-off slit.
In addition, it is desirable that the monochromator further comprises a photodetector for detecting the above-described measured beam, an exit slit which is disposed in the vicinity of the photodetector and on an incidence side of the measured beam and has a slit formed in the direction parallel to the rulings of the plane diffraction grating, and a second cut-off slit which is disposed in the vicinity of the exit slit and has a slit formed in the direction perpendicular to the rulings of the plane diffraction grating. Since the measured beam having passed through the exit slit and second cut-off slit detected by the photodetector, it is possible to ensure a further increased dynamic range by restricting the light-receiving range of the photodetector.